In the past, there have been proposed various power supply apparatus which simultaneously operates plurality of power devices to supply DC power from the power devices to one or more load devices connected to the power devices.
As an instance of the prior power supply apparatus, power supply apparatus including two power devices each configured to decrease monotonically its output voltage with an increase of its output current (see Japanese patent laid-open publication No. 10-248253). In this power supply apparatus, the two power devices shows individual output current-output voltage characteristics of which lines have different gradient from each other. This means that, when the two power devices varies their output current by the same extent, one of the power devices shows a variation of the output voltage different from that of the other power device.
In this power supply apparatus, each of the power devices operates to reach a balance point determined by its output current-output voltage characteristics and the load current in accordance with a magnitude of a total consumed current (load current) of all the load devices. Therefore, each of the power devices can output the desired output voltage and output current.
However, in this power supply apparatus where the two power devices shows individual output current-output voltage characteristics of which lines have different gradient from each other, the output voltages (that is, supply voltages for the load current) of each of the power device are varied due to a magnitude of the load current. Therefore, the power supply apparatus fails to output the stable supply voltage. In this power supply apparatus, in order to keep the supply voltage to the load device at a constant level irrespective of changes of the output currents of the each of the power devices, both of the power supply apparatus have to vary their output current-output voltage characteristics. For satisfying this requirement, the power supply apparatus needs to have a more complex configuration.
Now, in order to solve the above problem, there has been proposed power supply system including the power devices which are operated simultaneously. One of the power devices is configured to make constant voltage control, and the remaining is configured to make inclination control of outputting its output voltage of a DC voltage decreased with an increase of its output current. In this power supply apparatus, the power device performing the inclination control supplies a current to the load device while having its output voltage identical to an output voltage (reference voltage) of the power device performing the constant voltage control. In this situation, the power device performing the constant voltage current compensates for a shortage a current supplied to the load device. Therefore, according to the power supply apparatus, the supplied voltage for the load device is kept constant voltage (the output voltage of the power device performing the constant voltage control) irrespective of the variation of the load current. Consequently, this power supply apparatus can successfully supply power to the load device.
For example, a commercial power source is used as a power source connected to the above power device. The power device to be connected to the commercial power source includes a DC/DC converter. The DC/DC converter suffers from internal loss such as conduction loss (loss caused by an on-resistance of a switching element, a parasitic resistance of an inductor, or the like). As a result, a variation of conversion efficiency of the power device (a proportion of output power of the power device to input power of the power device) with an output current is expressed by a characteristics line having an output current maximizing the conversion efficiency, as shown in (a) of FIG. 9. With operating the power device connected to the commercial power source so as to supply an output current which is identical to a current supplied from the power device operating at the maximum conversion efficiency, it is possible to operate the power device efficiently.
However, in the prior power supply apparatus, each power device varies its output current depending on a magnitude of the load current. Therefore, the prior power supply apparatus is likely to operate inefficiently the power device connected to the commercial power.